1. Field of the Invention
The present invention relates to a technique for controlling a power supply, and more particularly for controlling a plurality of power supplies.
2. Description of Related Art
The increase in energy demand is believed to continue in the future to follow development of the global economy, particularly economic growth of the Asian regions. For example, the International Energy Agency predicts that global demand of primary energy in the year 2030 increases by 66% from that in the year 2000. Under these social circumstances, expectation for renewable energy such as solar energy, wind power energy, biomass energy, geothermal energy, and tidal energy is growing.
Despite its excellent properties of being clean and permanently usable, the renewable energy is pointed out to have a problem of unstable output. For example, with wind power generation, it is difficult to stably supply electric power because of a change in output according to natural phenomenon, that is, wind force. Needless to say, the change in output according to seasons or time zone is statistically predictable to some extent. However, it is essentially difficult to predict up to a short-term change in electric power according to an instantaneous change in wind force or wind direction. The renewable energy tends to cause an instantaneous fluctuation (hereinafter, “sharp fluctuation”) in the electric power. Therefore, it is difficult for the renewable energy to serve as a stable power supply.
A hybrid generation system that is a combination of a plurality of renewable energy sources such as wind power generation, solar photovoltaic power generation, and solar power generation has an advantage of being capable of stabilizing the whole output (Japanese Patent Application Laid-open Nos. 2003-339118, 2005-224009, and 2003-343416). However, as long as the hybrid power generation system uses the renewable energy as the energy sources, the system is susceptible to sharp fluctuation.
According to the method disclosed in Japanese Patent Application Laid-Open No. H4-372528, when power generated by wind power generation temporarily falls, a generator driven by a motor complements insufficient power (see paragraph [0012] or the like of Japanese Patent Application Laid-Open No. H4-372528). However, with the method disclosed in this conventional technique, there is no avoiding making the size of the motor and the generator larger as the size of the wind power generation is larger.